Sourcing Wooden Windows: Thermal Efficiency & Quality

[chris_d]

Dear readers,

I'm trying to source 7 new timber windows and have become a little frustrated at the smoke and mirrors put out by many manufacturers and suppliers.

There are some manufacturers eg Jeldwen who give potential buyers all the facts required to make an informed decision whilst others with supposedly reputable names (eg Everest) choose to hide the precise performance and quality characteristics of their products.

As I only have precise facts from just 2 window manufacturers: Premdoor and Jeldwen; I'm hoping that the readership might be able to suggest additional manufacturers given my requirements?

I'm still open to either softwood or hardward (oak) as surprisingly many of the higher thermal performance windows are only available in softwood and many manufacturers give substantially longer anti-rot/fungus guarantees on factory painted (hi-build) softwood frames than hardwood frames! However, if softwood, I want to be able to specify a RAL paint colour without MOQ constraints.

I'd like Lipped Casement (stormproof) windows without dividers or vents and with a whole window U-Value of 1.2 or less, employing DG or TG units with Argon/Krypton gas fill and low emissivity glass. I do not want ventilators as I'm aspiring to an air-tight build with MVHR. It would be advantageous to choose gasket/seal colours but I could compromise on that. Chrome, brushed steel, pewter or brass ironmongery.

If any window makers are reading this and wish to quote then feel free to contact me. I need 7 windows now and another 14 in the Autumn. Immediate sizes required are 630x1050 (1 off), 1200x1050 (1 off), 1770x1050 (5 off).

I officially hate window shopping.

TIA,
Chris

 

[Jacob]

..... surprisingly many of the higher thermal performance windows are only available in softwood and many manufacturers give substantially longer anti-rot/fungus guarantees on factory painted (hi-build) softwood frames than hardwood frames! .....

Not surprising at all. Depending on design details and manner of use - in general softwoods are more rot resistant than hardwoods. Also they tend to hold paint better than hardwoods.

 

[Dibs-h]

Is getting a local firm to make them for you - 1 for frames perhaps and another for glass not an option (or 1 for both)? Getting a 4-16-4 DG unit made up with Argon and using glass similar to Planatherm (or thereabouts) would get it to U value not very far from what you want.

Yes - it might not be "certified" - but beats window shopping.

Dibs

 

[chris_d]

Yes - it might not be "certified" - but beats window shopping.

Thanks Dibs, but as I've got the BCO looking over this they must be certified for Part-L although I'm ignoring the FENSA rubbish as I intend to self-install. When I sell up, I'll simply offer idemnity insurance to the buyer and their pedantic conveyancer.

Ta,
C

 

[Dibs-h]

Yes - it might not be "certified" - but beats window shopping.

Thanks Dibs, but as I've got the BCO looking over this they must be certified for Part-L although I'm ignoring the FENSA rubbish as I intend to self-install. When I sell up, I'll simply offer idemnity insurance to the buyer and their pedantic conveyancer.

Ta,
C

All my BCO cares about is the sticker on the glass that gives the U-Value of the pane (knowing that it will increase a little due to the frame) and as all mine state 1.3, don't have any issues.

All the glazing firms I've used - all "certify" that glazed unit - what above this is your BCO wanting? If he wanted it certified to BS:xxxx I could understand.

Dibs

 

[RogerS]

Chris..you have a PM.

It's not the whole window value but the centre-pane u-vale that must meet 1.2...easily achievable with 4:16:4 argon and planitherm total + as dibs says. The only issue you might have is if the ones you are taking out have ventilators then strictly speaking the replacements should. Likewise if they have special openers for fire egress. Nothing in the replacement should make things worse then what is being removed. Having said that the Herefordshire BCO's are refreshingly pragmatic and sensible.

 

[SBJ]

You have to be a bit careful with the centre pane method of calculation - look here - http://www.woodworkuk.co.uk/forum/viewt ... f=4&t=6591 to see it from a makers point of view (I'm Sainty by the way). It doesn't sound as though aesthetics are an issue? If the reason you want to achieve 1.2 is one other than that of satisfying your BCO then you will need either triple glazed units to get down to 0.8 ish or you might be able to get near with krypton - but I'm not sure. One other thing - warm edge spacer bars are another way of reducing the u-value of the overall window - they don't do much for the glass but the calculations take the spacer bar into consideration as it's a thermal bridge and the warm edge spacers have an impact on this.

If you have a dig around on there you will see that there appears to be durability issues with triple glazing.

I think the reason you are finding redwood rather than "hardwood" is that from everything that I've read, softwood windows are calculated using a 1.2/1.3 u value rather than a hardwood at 1.6 for the frame. The alternative, best of both worlds is Accoya which is 1.3 u value, with the durability of hardwood (I'm using it for the first time next week to make some windows so keep an eye out on the other site because I will probably post up my findings)- a quick google will tell you all you need to know.

If you want to have a chat about it, pm me your number and I'll talk you through what I know - it might be a short conversation!

 

[RogerS]

You have to be a bit careful with the centre pane method of calculation - look here - http://www.woodworkuk.co.uk/forum/viewt ... f=4&t=6591 to see it from a makers point of view (I'm Sainty by the way). It doesn't sound as though aesthetics are an issue? If the reason you want to achieve 1.2 is one other than that of satisfying your BCO then you will need either triple glazed units to get down to 0.8 ish or you might be able to get near with krypton - but I'm not sure. One other thing - warm edge spacer bars are another way of reducing the u-value of the overall window - they don't do much for the glass but the calculations take the spacer bar into consideration as it's a thermal bridge and the warm edge spacers have an impact on this.

Not sure if Chris is doing a new build/extension or replacement. At the end of the day, his best recourse is to speak to the guys at Hereford. Regarding the warm-edge spacer bars, I agree that if you are calculating the whole window (ie taking into account the heat loss through the frame) then they can help. However they do not help in trying to 'fiddle' the centre-pane u-value so you can use 4:12:4....that does not work.

I think the reason you are finding redwood rather than "hardwood" is that from everything that I've read, softwood windows are calculated using a 1.2/1.3 u value rather than a hardwood at 1.6 for the frame.

Spot on...softwood has better insulating properties than hardwood.

The alternative, best of both worlds is Accoya which is 1.3 u value, with the durability of hardwood (I'm using it for the first time next week to make some windows so keep an eye out on the other site because I will probably post up my findings)- a quick google will tell you all you need to know.

Which program are you using to calculate your window values?

 

[SBJ]

Not sure if Chris is doing a new build/extension or replacement. At the end of the day, his best recourse is to speak to the guys at Hereford. Regarding the warm-edge spacer bars, I agree that if you are calculating the whole window (ie taking into account the heat loss through the frame) then they can help. However they do not help in trying to 'fiddle' the centre-pane u-value so you can use 4:12:4....that does not work.

That's what I said.

Spot on...softwood has better insulating properties than hardwood.

That's what I said.

Which program are you using to calculate your window values?

I've done the calculations myself - but I will either use the buildcheck facility or St. Gobern have a program called calcuwin (or something like that) that you can download for free.

 

[chris_d]

Just so that casual readers don't get mislead: as of October 2010, windows in extensions to existing dwellings, should have a maximum whole window U-Value of 1.6 (doors <= 1.8 ) . The whole window U-Value is the rationalisation of the 'centre-pane' U-Value and the frame's U-Value using a specific approach (calculation).

My BCO has nothing to do with the U-Value that I'm aiming for, he was only brought into this discussion on the basis that he may need assurance that the windows have been made by an approved manufacturer who has had their design specification assessed for Part-L compliance irrespective of the actual U-Value that BRs currently specify.

My previous target whole window U-Value of 1.2 was asserted on the basis of a quick assessment of 'thermal performance bang for your buck' when you consider the permutations of DG v. TG, 24mm v. 28mm glazing units, Air v. Argon v. Krypton fill, metal spacers v. warm-edge spacers, softwood v. hardwood frames etc.

There is then another dimension of 'bang for your buck' when you overlay permutations of timber species, finish, guarantee periods etc.

From analysing further pricing data, I'd now assert that for softwood frames, a target whole window U-Value of 1.4 is likely to offer optimum value as this is the top end of what 24mm DG Argon filled units can achieve. Moving to 28mm TG Argon filled units gives U-Value of 1.3 (7% improvement) for ~25% price jump. However, 28mm TG Krypton filled units yields a U-Value of 1.1 (27% improvement) for ~61% price jump (relative to 24mm, DG Argon).

If you consider hardwood frames (specifically oak) then the cost increment of higher performance glazing is dwarfed by the timber species cost increment.

Now for completeness, proper cost analysis should also consider the long-term reduction in heating costs that higher performance glazing offers whilst also factoring the difference in maintenance costs between painted softwood and hardwood.

I officially hate window shopping....

 

[tomatwark]

It is because of the changes in the regs I decided to stop making windows.

Not so much because of getting them up to regs and getting the certification which is not to difficult to do.

But they are going to change the regs again in 2013 or 2016 and make it harder, but the main reason is for a one man band like myself it is going to cause more paper work and I guess when people sell their houses and have lost the certificates, I would have to issue replacements ( I know I would charge for this).

I can't be bothered with it any more.

There will be a market for someone making reproduction windows and not the generic stuff that the likes of Jeldwen make.

It is a load of b******s anyway as the sealed units lose the gas over a period of time becoming less efficient.

Tom

 

[chris_d]

It is a load of b******s anyway as the sealed units lose the gas over a period of time becoming less efficient.

Hi Tom,

What time period is that likely to be as my return on investment model is getting more complex by the minute?

Ta,
C

 

[tomatwark]

Hi Chris

I don't think it is that easy but my glass merchant says about 15 to 20 years. Other people say it is a lot longer, but as you have already found the companies will blind you with facts and figures to sell you their produsts

But it also depends on how the glass is installed, some people use glazing tape, some use silicone, but if the unit is not supported at the bottom it can cause it to fail and also use the wrong silicone and the chemicals in the silicone can cause it to fail.

I have always used glazing tape.

I took the PVCU windows out of my house 3 years ago and replaced them with DG gas filled sash windows and also topped up the insulation in my loft at the same time. My heating bills have not noticely increased even with the last 2 cold winters and my wife being at home on maternity leave last winter, so it was definately worth doing, the test will be when we get a milder winter and I can compare the gas usage between the last mild winter.

But on the whole it was definately worth doing and I don't have nasty plastic windows any more.

My comment about the gas leakage is more to do with the build regs as the window only meets them for a certain time, and all it is the goverment trying to meet their global warming targets.

Tom

 

[Dibs-h]

Chris

http://www.hodgsonsealants.com/techlibdoc.php?p=5
http://www.hodgsonsealants.com/techlibdoc.php?p=6

point you to 2 documents for installation of DG units in timber windows.The main page is here - http://www.hodgsonsealants.com/html/technical-library/

Some folk prefer to "ventilate" the bottom of the beading - but part of me wonders whether that's actually a "fix" for poor installation and if the DG units had been installed as per above, whether there would be any issues. Some of my DG units are coming up to 8 or so years and no issues so far, having been fitted as per the 1st link (B6).

It does get a little interesting if it's self clean glass - which our upper floor units are - as the sealant can't be silicone and isn't cheap. but other than that no probs.

HIH

Dibs

 

[RogerS]

....
Some folk prefer to "ventilate" the bottom of the beading - but part of me wonders whether that's actually a "fix" for poor installation .....

No, it's so that if the method of fixing should fail then any water getting in between dgu and frame can run away. If it doesn't then any pinpricks that develop in the hotseal will draw up the standing water into the dgu...thus condensation. There's a stack of research documents out there on the web.

 

[Dibs-h]

....
Some folk prefer to "ventilate" the bottom of the beading - but part of me wonders whether that's actually a "fix" for poor installation .....

No, it's so that if the method of fixing should fail then any water getting in between dgu and frame can run away. If it doesn't then any pinpricks that develop in the hotseal will draw up the standing water into the dgu...thus condensation. There's a stack of research documents out there on the web.

When I was looking to do my windows before - I spent an inordinate amount of time on Google and the only reference that I could find was one particular chap's website (who's name escapes me at the mo), no other website, no evidence\research or any standards, etc.

I chose to use a fully bedded method - and looking at the approvals, NHBC, BS, etc. if done correctly - I don't feel there would be any issues. But as with all things - horses for courses.

Dibs

 

[RogerS]

Here's one link

http://www.springerlink.com/content/v3152h568601871v/

Like all things Google...depends on your search terms...and luck!

 

[Dibs-h]

Here's one link

http://www.springerlink.com/content/v3152h568601871v/

Like all things Google...depends on your search terms...and luck!

Any more links Rog?

That one, the article doesn't appear to comment on the actual design or DG units themselves, etc. It's comments appear to be related to poor, this poor that - i.e. poor quality materials, etc. I've had units made directly by Pilkington themselves and my usual suppler, whose productions lines and materials, I've seen. Their sealants are all supplied by Bostik and issues with dew points in the cavity gas (be it air or whatever) aren't relevant when the gas has been replaced with argon.

As I stated earlier - I don't see any issues with fully bedded DG units in timber frames as long as they are done correctly, using quality materials.

I appreciate folk may feel differently.

Dibs

 

[Austin Branson]

As I stated earlier - I don't see any issues with fully bedded DG units in timber frames as long as they are done correctly, using quality materials.

I appreciate folk may feel differently.

Dibs

Dibs, sorry to be so stupid, but what do you mean by that? What is 'fully bedded'?
Austin

 

[themackay]

I am in the process of building an extension windows were fitted about 2 years ago I bought them from Nordan redwood frames tripple glazed if I am correct U value for the whole window was less than 1 the frames also have a insulation layer sandwiched in the frame.I thought these were the best I could find at the time for enefficiency versus cost.Not cheap but worth a look